Soluble Troger’s base-based polybenzimidazoles containing naphthalene units with improved phosphoric acid tolerance for use as high-temperature proton exchange membranes

Most Troger's base (TB)-based polymers reported for high-temperature proton exchange membranes (HT-PEMs) exhibit low phosphoric acid (PA) tolerance due to excessive swelling in PA solution. In this work, a novel dicarboxylic acid monomer containing both naphthalene and TB units (TB-N-COOH) was successfully synthesized, and the corresponding TB-based PBI homopolymer (TB-N-HPBI) was prepared for HT-PEM fuel cells (HT-PEMFCs). In addition, naphthalene units were incorporated into TB-based PBI copolymers (TB-N-CoPBIs). The resulting TB-N-HPBI and TB-N-CoPBIs exhibited high molecular weight and excellent solubility in organic solvents (e.g., DMSO and NMP) because of the twisted TB units. The incorporated naphthalene units significantly improved the oxidative stability. More importantly, the existed naphthalene units in polymer backbones endowed the membranes with excellent PA tolerance capability because of the rigid structure and poor affinity to PA, which limited excessive swelling in 85 wt% PA solution and hence promoted acid uptake capability. Among of them, the TB-N-HPBI membranes showed ultrahigh PA uptake of 508.6%, and resulting in the proton conductivity of 201 mS cm−1 at 160 °C under non-humidity conditions, which are far superior to most reported TB-based polymer membranes. In addition, the maximum peak power density (H2/air) of 437.2 mW cm−2 was achieved. Our present work demonstrated that the incorporation of the naphthalene units into TB-based PBI significantly promoted the improvement of comprehensive performance for HT-PEMFCs.